Adding a Screen-Grid Valve to the Hammarlund-Roberts Four

Radio Record, Volume V, Issue 14, 16 October 1931, Page 14

Adding a Screen-Grid Valve to the Hammarlund-Roberts Four

A Simple Operation that will Greatly Improve the Set

By the

Technical Editor.

TERE are a large number of four-valye Ianmmarlund-Rob-erts sets scattered throughout the country, and many owners of these have requested a sketch ‘and particwars of how to add a screen-grid valve, To supply all these individual inquiries would mean thuat we would have to go beyond the scope of "Questions and Answers," so there is only one thing to do, and that is to describe, in a special article. the circuit to be used. It will, be necessary to extend the set slightly, and this can be done: in either of two-ways. One is by making the adapter’ into a type of boosternot quite a regular booster, for these things are rarely satisfactors-put into a special type, and keeping t more or less separate, though in juxtuposition to the set. The other and better plan is to dismantle the panel, extend the baseboard to the left, and. rearrange the panel slightly in order to obtain symmetry, A certain amount of shielding must be introduced, and this preferably should take the form of. three sides of a: box, such as was deseribed for the "Kestrel Three." This will be adequate and will prevent the set from going into uncontrollable oscillation. After considerable experimenting we have come to the conclusion that the circuit shown on this page is the best for this particular hook-up. It will

be seen that in reality there are very few changes to be made in the ordinary ELR. circuit. Another coil will be necessary, and this must follow on the general lines of the other coils used in the circuit. It will be noticed, too, that a system of tuned anode is used. This does away with the separate primary which, with a 3in. coil, wound on celluloid, as are the H.R. coils, would present certain difficulties in the making. By

using a tapped coil, the impedances are more or less automatically marched and high gain can be obtained. Another interesting feature is the method of volume control. by adjusting the screen-grid voltage. The use of a high value potentidmeter does away with the need for an extra tapping on the battery. but its use imposes a certain drain on the batteries. This, although less than 1 m. amp. in magnitude, will, if draining continuously, shorten the life of the battery, certainly only a little, but it will be shortened. So it is suggested that. when the set is not used for long periods, the "B-+-" r.f. connection be removed. Simply disconnecting "B-’ would have the same effect. Where an eliminator is employed the cireuit is perfectly satisfactory in every ‘respect. . , Certain Difficulties. IN presenting a layout sketch there are certain difficulties with which one must contend. In the first place. no two sets, unless they are made from blue-prints,-will be exactly the same, so we have used as a basis the layout suggested in the 1930 "Radio Guide." This, at least in the r.f. stage, differs very little from the commercial kit set, and possessors of the latter should have no difficulty in making the suggested alterations, These, by the way, are remarkably simple, and anyone who has not had previous experience of constructing should have no difficulty in connecting up the extra stage, A list of components is given, and it is necessary; before one commences, to see that all these are present. The Coil. HERE are the specifications for the "special coil: You will need a threeinch celluloid former (cardboard, exe-

lon or ebonite would do). ‘The coil is to be wound with 22 d.s.c. und the winding length will be 1 7-Sin., representing 58 turns, but the number of turns must be the same as on the secondary of the existing aerial coil. A sketch of this is reproduced on’ th next page, so that there can be no mistake in the number of turns that are to be wound on the special coil. If you are using any other type of coil in this position, the new coil must agree with it. Three tappings are taken out as the winding proceeds, ‘The exact positions are not important so long as three ave spread well over the winding length. The beginning and the end of the special coil are at the bottom, while the three tappings are on the top. This is made quite clear by the diagram. If celluloid formers are used, two strips of thick celluloid, one above and one underneath the main coil, will serve fo support a thin strip of chonite upon which the solder lugs : can be fastened. To these solder lugs are attached the wires going to the tappings. You can, of course, get this coil specially made. Do not forget to ascertain the number of turns now on the secondary of your aerial coil

and make certain that the condenser, which is to go in the first stage, is exactly the same as the condenser now used in the first stage. If you do not know the size, describe it to your dealer as well as you can and’ count the number of plates. Once you have the special coil, this is what you do, We shall presume you intend to dismantle the panel. Lake off the panel and arrange the fittings on the new panel so that they are more or less symmetrical You

Parts Required One special coil, One .0005 tuning condenser, If you are using .00035 in your set now, your new one should be the same. One dial; one UX. socket, One screen-grid valve. One r.f. choke; one .0008 condenser, Two 2 mfds condenser, 250 working. One 5 ohms filament resistance. One 100,000 ohms __potentiometer. N A panel 6in. longer than the’ one at present in use. A piece of baseboard 6in. x the depth of the existing baseboard, A metal sereen, bottom 6in. x the depth of the baseboard, side depth of the. baseboard by the height of the panel, front 6in, x the height of the panel,

-_----- will have the parts that are on the k original panel, plus a dial and knob. It is suggested that the three dials be now evenly spaced and the knob be put on a level with the r.f. rheostat. Just be careful, .if you rearrange the condensers for the remaining part of the set, that you do not get them so that they will not fit in with the wires that have been removed from them. Rather than cause any complications, merely line up the condenser for the screen-grid stage and volume control and do not attempt to alter the spacing. Slip the panel back before going any farther, Extend the baseboard to take up the Gin. of space now left at one end. Take your shield and fasten it by means of wood screws to the baseboard. You now have a metal shield separating the screen-grid stage from the rest, and this shield extends along the base of the new compartment and up the panel and back. . Laying Out the Extra Stage. UN SOLDER the connections going to the grid of the first r.f. valve and the aerial condenser from the existing aerial coil. Do not take these wires off the valve and condenser. Disconnect the wire which goes from the

earth termina! through the baseboard to "A-." Now remove this coil and the small strip with the aerial and earth terminal into a corresponding position in the new compartment. Fasten it in position, taking the connection that previously went from the earth terminal underneath the base ‘board now to the metal shield itself. Connect the top of the secondary coi! with the fixed plate of the tuning condenser and the grid to the screen-grid valve holder; Connect the "A-" wire, which was left homeless with the removal of-the aerial coil, to the shield.. The other components are ted into position as is shown in the diagram and the wiring carried out as indicated. The new coil must be placed in the position formerly occupied by the aerial coil. -The connection from the plate of the screen-grid valve is taken to the most. suitable tapping on this eoil. Connections can be made most easily by a small battery clip. The top of the coil goes to the grid of the rf. valve and the fixed plate of the

tuning condenser. The other end goes to earth. Notice that the top of the valvethat is, the anode in the case of the English and Continental valves-passes through the shield without touching it, to the .0008 condenser... Also that a connection from the volume control goes through the shield. and down

underneath the baseboard to "B-+-" r.f. This point is clearly shown on the diagram of the original H.R, Four, also reproduced. j For the sake of clearness we have shown wires in the. original set, which

are not interfered with, by a broken line, while new wires, which go under the baseboard, are shown with a different type of broken line. The full line indicates the new wires. Screw heads indicate that that wire is connected with the metal base. You may, if you wish, use the American type of screen-grid valve, either the 222 or 230. If you use either of these two valves, remember that the connections to the sereen-grid valve are slightly different. The grid is on the top in the case of American valves, and the wires now going. to the grid terminal of the valve socket must go to the top of the valves. The vacant grid terminal must take the. wire now going to "P" of the valve socket, while "p" takes the connection which goes through the screen to the .0003 condenser. , It is possible that some constructors will wish to try the differential reaction condenser in a Hammarlund Roberts set. Of course, if they do this, the set is no longer a Hammarlund Roberts. It is not suggested that this will be any better. The constructor may have some considerable difficulty in getting the set to oscillate without redesigning the regeneraformer.